Water Heater Sizing Guide

Selecting
the best products for water heating

Water heater sizing guide article is about choosing the right
water heater size using an expert guidelines, recommendations and tips.

Water heating is the second
largest energy
user in your home and with the rising cost of gas, electricity and
other fuels, you should be careful when selecting the size of your
water
heater.

General sizing rule: The heater should provide sufficient hot
water at a
temperature of min. 120 F at the busiest time of the
day.

Keep in mind that oversized heater means wasted money and
energy
as you will be heating more than you need. The heat and energy
will be lost through the tank walls and pipelines and every time
unnecessary quantity of water has to be re-heated.

An undersized heater means problems.

This is why this water heater sizing guide exists; to provide
you all the necessary information, we think you need, when
selecting and buying a heater.

Sizing
Guide Tips

When you buy a heater it is normal to expect that it will
perform
perfectly and
deliver hot water whenever we need it, day, night, year by year.

There are many different sizes, from small
point of use and medium tankless water heaters to large tank models and
boilers for
higher demand.

Tankless heaters, for example, are providing hot water on
demand,
continuously, and
its output can be expanded by applying the multi
system and by connecting several units into one system.

Tank type heating units are different; they are large in size
and
occupy much more area from the floor space.

The tank-type heater must be sized properly for your home and
family
needs and also to meet demand for variety applications,
including showers, dishwashing and washing machines.

Undersizing
a Water Heater

An undersized heater automatically doesn't provide
enough hot water and it may cause overheated water. It may also
result in condensation and pilot flame
outage due to drips onto the flame.

Common symptoms of the undersized heater are:

Not enough hot water

Heavy condensation

Rust chips on the burners and flue tubes

Premature tank failure

Improper
sizing can create problems that will reduce the life of the heater,
so units that are undersized are not eligible for warranty
consideration.

On the other side, purchasing a properly sized tank it will minimize
burner or other element operation
when small quantities of hot water is used, so there will be less
stress on them.

Important
Factors

Probably
the most important two factors are storage (capacity) and heat input.
It is
important to have the right water heater size especially when large
quantities of hot water are required in a short period of time.

On
the other side if the peak usage period is for an extended period of
time (more than two hours) than the heater recovery rate is
more
important.

Another tip from experts is to buy a heater with the highest
Energy Factor (EF). EF measures the
efficiency of the heater. They are more expensive, but with
higher EF you will purchase EnergyStar models, save on energy and get
the rebate through government incentives.

With larger storage tank models you will get the lower EF
and lower EF gets you the higher operating costs.

What
else to consider

When selecting a water heater, it is important to take into
account the
size limitation like diameter and height of the unit, as there are
models like short, tall, lowboy, table
top... so it can perfectly fit inside the designated room.

Here are suggestions and major factors that directly influence
water usage, to investigate and include when selecting
a heater:

What is the size of your home and family?

How many bathrooms, tubs, dishwasher, washers you
have... any additions in the future?

Age of family members, kids, teenagers... family is
growing?

What is the pattern of the usage?

Quantity and type of fixtures.

Pattern
of the usage is very important as it gives you the idea of the
peak hour demand or when is the busiest hour of usage (i.e. peak time
might be in the morning between 6 and 7 before parents go to work and
kids to
school).

Here are the consumption rates as an indication of how much
water are
required by certain application:

For shower or bath you need around 20 gallons.

Showers with energy efficient shower head - 2.5-3
gal/min.

Automatic dishwasher needs around 14 gallons.

Washing machine needs 32 gallons to wash the clothes.

To shave you need approximately 2 gallons.

For storage tank-type heaters if there is a need for more hot water
than what your existing conventional unit provides, you may want to
consider buying high recovery unit or split the system and install two
units.

Sizing and
the First Hour Rating

Don't make a mistake like majority of consumers to buy
a water heater based on the size of the storage tank. The peak hour
demand capacity or First Hour
Rating
is more important. This information can be found on the Energy
Guide label or brochure.

First Hour Rating is an indicator of how much hot water, one
unit
can provide. For natural gas, for example, first hour ratings range is
from 40 to over 100
gallons.

It is shown in gallons and is the result of the
combination of the amount of usable hot water stored in the tank and
how many gallons of usable hot water, the heater can generate in
one hour of operation.

Example: If 40-gallon natural gas heater stores 31 gallons of
usable hot water and it delivers
41
gallons of usable hot water in the first hour, its First Hour Rating
is 72 gallons.

High First Hour Rating allows you to buy a smaller
model
as it will result in good performance even during the peak time and
high demand while minimizing the cycle and standby losses.

The
most common sizes

The most used tank size (based on the common family size
of four to six) and general rule for water heater sizing are:

30
gallons WH - enough for one bath
residential homes or apartments.

40
gallons WH - for two
bath residential homes or two applications.

50
gallons WH - for
three bath residential homes or three applications.

The above is the general guideline but if want to use the
electrical
heater for example, for family of four, and you live in
a home with
two full bathrooms, and using washing machine and dishwasher
you should be within 50 to 80 gallons tank capacity range and
around 5.5 kW of power produced by heating elements.

For the same requirements, and if using a gas, recommendation
is to use
a 50-gallon tank with 40,000 BTU/hr gas input.

Here are the assumptions you should also consider for water heater
sizing:

Electric heater is considered to have close
to
the 100% recovery
efficiency due to its immersion installation.

Gas heaters are considered to have 75%
recovery
efficiency, which means that 75% of the total heat produced by the
burner is absorbed by the water in the tank. 25% is energy waste.

Keep
also in mind that around 70% of the hot water is drawn before dilution
by the incoming cold water. For example, 50 gallon tank
will deliver 35 gallons of usable hot water.

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